The present invention relates to an eddy current probe that is particularly designed for inspecting the condition of conduits during maintenance of steam generators in nuclear power plants, said heat exchangers comprising a ring formed of a large number of elbow-shaped tubes in which the heat-exchanger fluid flows.
This type of inspection must be carried out rapidly, requiring the probes to move through the tubes at relatively high speeds of approximately 500 mm/s. The flow-sensitive sensors located at the tip of the probe must therefore be constructed and positioned to give sure, easy detection.
Several devices have been proposed in the prior art. For example, French patent 2,668,605 gives a detailed description of the chief components of a probe and mentions the possible use of ultrasound of eddy current sensors for carrying out the inspection without, however, giving much information on the best way of arranging the sensors. In contrast, U.S. Pat. No. 5,256,966 describes a probe that only uses eddy currents. Eddy current sensors are fitted with sensing coils through which an AC current is passed, creating a magnetic field around the coils, and receiver coils connected to a circuit of which the impedance is measured. This impedance is governed by the eddy currents produced by the magnetic field in the neighboring conducting material, i.e. inside the material constituting the tube, and the impedance varies when flaws in the tube cause local increases in resistance and modify the way the eddy currents flow. The flaws in the tube are thus identified by variations in impedance on the terminals of the receiver windings.
The coils of U.S. Pat. No. 5,256,966 are both transmitters and receivers and are either positioned on a single ring or, where space is an issue, placed alternately on two neighboring rings. This is done with the aim of transmitting a magnetic field over the entire circumference of the tube in order to detect flaws which would go undetected if there were spaces between the magnetic fields created by individual coils. This arrangement, in which a plurality of sensors is used, is made possible using a switching device that makes it possible simultaneously to activate only some of the sensors, together with a multiplexing device which is used to transmit by turns data from one series of sensors along each measuring wire.
A different design is described in an article by Sullivan et al. entitled xe2x80x9cDetection coverage of transmit-receive eddy currents array probesxe2x80x9d that was distributed during the 14th EPRI Steam Generator NDE Workshop held in Seattle between Aug. 7 and 9, 1995. This system uses four magnetic field-emitting coils placed around the circumference of the probe together with two rows of four receivers each of which is positioned on either side of the row of transmitters and surrounding the transmitter coils. In this design the transmitters and receivers are thus separate coils and each transmitter is surrounded by four receivers that are sensitive to the eddy currents it induces. Measurements are differential, meaning that the result produced on the receivers of one row is subtracted from the result of the respective receivers of the other row.
Known probes all have drawbacks as concerns the detection of flaws. The probe described in the Sullivan article is effective in detecting transversal cracks, particularly the ends of such cracks, but is insensitive to longitudinal flaws. The probe described in the US patent has the drawback of being insensitive to certain flaws, particularly longitudinal flaws located mid-way between neighboring coils where the fields produced by adjacent coils balance one another out. More surprisingly, it is also insensitive to flaws whose edges lie mid-way between adjacent coils. U.S. Pat. No. 5,506,503 describes how, within a probe having a row of alternately transmitting and receiving coils, certain transmitter coils may be disactivated one by one to detect additional flaws. This statement would suggest a need to scan the part being inspected several times with different transmitter coils activated during each scan, which is clumsy and time-wasting.
The aim of the present invention is both to detect all the flaws, particularly cracks, in the surface being inspected, irrespective of their orientation. The preferred embodiments of the invention are satisfactorily sensitive to all types of flaw and the part to be inspected is always covered in a single scan.
The invention is based on using an array of simultaneously active transmitting and receiving coils that does not vary in shape and that is moved across the width of the part being inspected, either by moving the probe (transversal and particularly using a rotating movement for a circular probe) or by switching the coils. The receiver coils in the array are subjected to different magnetic fields and are thus sensitive to different flaws in the part being inspected.
The invention relates to a probe for inspecting a conducting part characterized in that it comprises:
a body fitted with at least one row of identical pairs of adjacent transmitting and receiving coils;
means for activating the transmitting coils by energizing them by means of an AC current of sufficient amplitude to create a magnetic field that extends as far as the receiver coils of the neighboring pairs in the row to those belonging to the energized transmitter coils;
means for activating the receiver coils while measuring signals induced in them by the magnetic field;
and means for displacing a coil array composed of energized transmitter and receiver coils in the direction of the row, said array comprising at least two transmitter coils and one receiver coil, the coils of the array remaining in the same relative positions.